1. Field of the Invention
The present invention relates to a tone spacing and power level monitoring arrangement for FSK lightwave systems and, more particularly, to a polarization-independent, fiber-based monitoring system which utilizes a self-heterodyning technique.
2. Description of the Prior Art
In FSK (frequency-shift-keyed) transmission systems, the carrier frequency .omega..sub.s is modulated by a binary data stream such that the carrier frequency shifts between two distinct values related to a transmitted logic 0 (.omega..sub.s -.omega..sub.d) and and a transmitted logic 1 (.omega.+.omega..sub.d). Tone spacing (also referred to in the art as frequency-shift spacing) is defined as the difference (i.e., 2.omega..sub.d) between these two transmitted frequencies. In conventional microwave electronic FSK systems, tone-spacing will remain relatively constant. However, in lightwave FSK systems, the carrier frequency may experience drift in response to a number of factors, including temperature, age, and data rate. Therefore, the tone spacing in lightwave FSK systems may also drift. The performance of the receiver in lightwave FSK systems, in terms of recovering the transmitted data signal, may then be affected by any change in tone spacing.
One prior art technique for providing control of tone spacing is disclosed in U.S. Pat. No. 4,700,352, issued to M. Shikada et al. on Oct. 13, 1987. As described therein, a self-heterodyne technique is used to extract the beat frequency between the transmitted signal and a delayed version thereof. The beat frequency corresponds to the tone spacing value and, therefore, any variation from the desired tone spacing value will result in a change in beat frequency. This change in beat frequency is translated into an error signal which is then passed back to the transmitting device. Adjustments in operating parameters (e.g., bias current, operating temperature) are then made to the laser until the beat frequency and predetermined tone spacing are essentially identical.
Although the Shikada et al. arrangement seems feasible in theory, in application there exist a number of drawbacks. Firstly, as disclosed, Shikada et al. require the use of polarization-maintaining components (fiber, splitter, coupler) to perform the self-heterodyning operation, where such components are not considered as practical alternatives at this point in time. Additionally, Shikada et al. require precise control of the signal delay such that the input lightwave signal is shifted exactly one bit (or two bits in an alternative embodiment). The ability to control such a precise delay is uncertain and further limits the use of the device to a set data rate. That is, any change in the transmitter data rate would necessitate a comparable change in the length of delay in the tone-spacing control arrangement.
Therefore, a need remains in the prior art for a means of controlling the tone-spacing in lightwave FSK systems which is relatively robust and is neither polarization nor data rate dependent.